1. Field of the Invention
The invention relates to fiber optic transmission systems, in particular wavelength division multiplex fiber optic transmission systems, and more precisely to regeneration in wavelength division multiplex fiber optic transmission systems.
2. Description of the Prior Art
Regular synchronous modulation of signals has been proposed in wavelength division multiplex fiber optic transmission systems, preferably optical modulation, especially in high bit rate systems. Various methods have been proposed for synchronizing the various channels at the time of regeneration, including allocating wavelengths assuring synchronization at regular intervals on the link, applying time-delays, modulating at a frequency that is a multiple of the signal frequency.
The modulation can be intensity modulation and/or phase modulation.
The paper by F. Devaux et al., “20 Gbit/s operation of high efficiency InGaAs/InGaAsP MQW electroabsorption modulators with 1.2 V driven voltage”, IEEE Photonics Techn. Lett., vol. 5 pages 1288–1290 (1993), describes intensity modulation with narrow filtering using an electroabsorption modulator.
The paper by M. Nakazawa et al., “Experimental demonstration of soliton data transmission over unlimited distance with soliton control in time and frequency domains”, Electronics Letters, vol. 29 No. 9, pages 729–730, describes another form of intensity modulation; the document proposes the use of an LiNbO3 Mach-Zender intensity modulator. The paper by Leclerc et al., “Polarization independent InP push-pull Mach-Zender modulator for 20 Gbit/s solitons regeneration”, Electronics Letters, vol. 34 No. 10, pages 1011–1013 (1998), describes an InP Mach-Zender intensity modulator.
Phase modulation by the crossed Kerr effect between the signals transmitted and a clock propagating in a Kerr fiber is known in the art. The paper by S. Bigo and O. Leclerc, “Fundamental limits of all-optical synchronous phase regeneration through Kerr fiber”, Proceedings d'ECOC'97, p. 311, Edinburgh, 22–25 September, describes one example of the use of this kind of technique; the paper shows that phase modulation by the Kerr effect can be effective, even in the presence of noise or slip between the signals transmitted and the modulation clock. The paper by O. Leclerc et al., “2×20 Gbit/s, 3500 km regenerated WDM soliton transmission with all-optical Kerr fiber modulation”, vol. 34 No. 2, pages 199–201 (1998), shows the feasibility of phase modulation by the Kerr effect in wavelength division multiplex transmission systems.
Phase modulation and intensity modulation in a regenerator are also known in the art. The paper by P. Brindel et al., “Black-box optical regenerator for RZ transmission systems” Electronics Letters, vol. 35 No. 6 (1999), pp. 480–481, proposes separate intensity modulation and then phase modulation. The paper by B. Dany et al., “Transoceanic 4×40 Gbit/s system combining dispersion-managed soliton transmission and new “black-box” in-line optical regeneration”, Electronics Letters, vol. 35 No. 5 (1999), pp. 418–420, applies the same regeneration technique to a four-channel transmission system using controlled dispersion soliton propagation with separation of channels and one regenerator per channel.
FR-A-2 759 830 and French patent application 99 14117 filed on 10 Nov. 1999 (PCT/FR00/03124), “Channel synchronization by dispersive fiber in a wavelength division multiplex transmission system”, propose synchronous regeneration by intensity modulation followed by phase modulation in separate modulators.
U.S. patent application Ser. No. 09/000,451 proposes using a non-linear optical mirror in a loop with two control inputs for phase modulation of optical signals. It refers to a non-linear optical mirror with two control inputs and is therefore the fiber equivalent of a Mach-Zender modulator with two electrodes, and allows independent control of the intensity and phase modulation depth.
The above solutions have disadvantages. Separating phase modulation and intensity modulation is a complex and therefore costly solution. The non-linear loop mirror technique does not allow control of accumulation of noise or dispersive waves and this conjoint modulation solution limits the total length of the transmission system.
The invention proposes a solution that overcomes these various disadvantages; it proposes a regenerator using intensity modulation and phase modulation with a minimum of hardware components and simple operation.